Degradation of hexabromocyclododecane (HBCD) by nanoscale zero-valent aluminum (nZVAl)

Chemosphere 244 (2020) 125536

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Degradation of hexabromocyclododecane (HBCD) by nanoscale zerovalent aluminum (nZVAl) Yuting Jiang a, c, Shiying Yang a, b, c, *, Junqin Liu c, Tengfei Ren c, Yixuan Zhang c, Xinrong Sun c a b c

Key Laboratory of Marine Environment and Ecology, Ministry of Education, Qingdao, 266100, China Shandong Provincial Key Laboratory of Marine Environment and Geological Engineering (MEGE), Qingdao, 266100, China College of Environmental Science and Engineering, Ocean University of China, Qingdao, 266100, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


HBCD degradation by nZVAl is one of the highest efficiency methods.
Nearly 100% debromination can be obtained under optimized conditions.
HBCD was degraded to DBCD and then CDT through reductive debromination.
The oxide film on the surface of nZVAl was corroded during the reduction reaction.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 July 2019 Received in revised form 1 December 2019 Accepted 2 December 2019 Available online xxx

Hexabromocyclododecane (HBCD) has been listed in Annex A of the Stockholm Convention on Persistent Organic Pollutants (POPs) in 2013, but till now there is a lack of efficient methods for its degradation. In this study, nanoscale zero-valent aluminum (nZVAl), an excellent reductant with a very low redox potential of E0(Al3þ/Al0) ¼ 1.662 V and strong electron transfer ability, was used to reductively degrade HBCD. Nearly 100% HBCD was degraded within 8 h reaction at 25  C in ethanol/water (v/v, 50/50) solution without pH adjustment. And about 67% cyclododecatriene (CDT) was obtained, which is the complete debromination product. What’s more, the yield of Br could achieve nearly 100% after optimizing conditions. The reaction was strongly promoted by increasing the dosages of nZVAl or decreasing the initial concentration of HBCD. The temperature had the most significant influence and the degradation was completed in 40 min with elevating the reaction temperature to 45  C. The reaction mechanism was further revealed through the characterization of nZVAl particles before and after the reaction by SEM-EDS, TEM, HRTEM, XRD, and XPS. It was found that, after corrosion of the oxide film on the surface of nZVAl, metallic aluminum inside was exposed. The reactive sites were provided and electrons released were transferred from nZVAl to HBCD, causing HBCD degraded to dibromocyclododecadiene (DBCD) and then CDT by reductive debromination. These findings imply that nZVAl can degrade HBCD efficiently with no extra energy input and this offers a new idea for better treatment of HBCD. © 2019 Elsevier Ltd. All rights reserved.

Handling Editor: Jun Huang Keywords: Hexabromocyclododecane (HBCD) Nanoscale zero-valent aluminum (nZVAl) Cyclododecatriene (CDT) Reductive debromination Reaction mechanism Surface film corrosion

* Corresponding author. Key Laboratory of Marine Environment and Ecology, Ministry of Education, Qingdao, 266100, China. E-mail address: [email protected] (S. Yang). https://doi.org/10.1016/j.chemosphere.2019.125536 0045-6535/© 2019 Elsevier Ltd. All rights reserved.

1. Introduction Hexabromocyclododecanes (HBCD) has become the third

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widely used brominated flame retardant (BFR) throughout the world, following polybrominated diphenyl ether (PBDE) and tetrabromobisphenol-A (TBBP-A) (Marvin et al., 2011). It is widely used in polystyrene electrical equipment, upholstery textiles and so on. And it is primarily applied in expanded polystyrene and extruded polystyrene foam for thermal insulation in buildings (Cao et al., 2018). As an additive flame retardant, HBCD can enter the environment by a lot of different pathways, for example, emission during production, leaching from consumer products, or following disposal (Covaci et al., 2006). HBCD has been found in all environmental media including air, water, sediment, biota (Cao et al., 2018; Lara et al., 2018). Even in breast milk and blood, the presence of HBCD has been evidenced (Fromme et al., 2016; Lu et al., 2018). Toxicity tests indicated that HBCD has endocrine disrupting effects, neurotoxicity, hepatotoxicity, immunotoxicity, reproductive and developmental toxicity, and may induce cancer (Hong et al., 2017). Due to these concerns and its long-term persistence, high bioaccumulability, and potential toxicity, HBCD has been listed in Annex A of the Stockholm Convention on Persistent Organic Pollutants (POPs) in 2013. In January 2018, China added HBCD to its list of priority substances, which may imply restrictions in productions or limitations of discharges. Thus, it is crucial to remove HBCD from the environment. The research on HBCD degradation was developed slowly because HBCD is difficult to be decomposed. Several methods have been applied, including physical adsorption (Li et al., 2017a, 2017b), biodegradation (Le et al., 2017; Heeb et al., 2017) and abiotic degradation, such as photolytic degradation (Zhou et al., 2014; Yu et al., 2015), thermal degradation (Wu et al., 2019) and mechanochemical degradation (Zhang et al., 2014; Yan et al., 2017). In addition to these ways, inorganic minerals with reducing capacity were also used for reductive debromination of HBCD. For instance, polysulfide and bisulfide (Lo et al., 2012; Zhang et al., 2019), zerovalent iron nanoparticle aggregates (Tso and Shih, 2014), iron monosulfide (Li et al., 2016), and sulfidated nanoscale zero-valent iron (Li et al., 2017a, 2017b). Although these methods have been tried, a way with high efficiency for HBCD degradation still needs to be explored. Zero-valent metals have a wide range of applications in the degradation of contaminants, especially for zero-valent iron applications (Ling et al., 2018). It is worth noting that, zero-valent aluminum (ZVAl) has been gradually drawn much attention for environmental contaminants removal in recent years (Yang et al., 2016; Arslan-Alaton et al., 2018; Nidheesh et al., 2018). A far greater thermodynamic driving force for electron transfer can be provided by ZVAl (Bokare and Choi, 2009) because of its more negative standard reduction potential (E0(Al3þ/Al0) ¼ 1.662 V) than zero-valent iron (E0(Fe2þ/Fe0) ¼ 0.43 V). In the process of reduction, contaminants capture electrons and were reductively transformed by ZVAl (Ren et al., 2018). Surface corrosion was detected during the removal of contaminants using ZVAl (Yang et al., 2017). The dense surface oxide films became loose in the process of surface corrosion (Gai et al., 2012), and this is beneficial for the metallic aluminum inside to release electrons. Our prior studies have proved that ZVAl has a powerful ability to remove many kinds of contaminants, such as chromate (Zhang et al., 2018), nitrobenzene (Yang et al., 2017), trichloroethylene (Ren et al., 2018) and so on. Attempts have been made for PBDE degradation by ZVAl (Yang et al., 2018), but there is no information available for the reducing capacity of ZVAl towards HBCD. Therefore, it is particularly necessary to ascertain the possibility and the mechanism of HBCD removal by ZVAl. In this work, nanoscale zero-valent aluminum (nZVAl, ~50 nm) was selected as the reductant, and the process of HBCD degradation was explored systematically. First of all, HBCD can be degraded

efficiently under various conditions, including dosages of nZVAl, oxygen, temperature, inorganic ions, and initial pH. And then, the pathway of degradation was given regarding the analysis of degradation products using gas chromatographic-mass spectrometry (GC-MS). Finally, characterizations of nZVAl before and after reaction were carried out by SEM-EDS, TEM, HRTEM, XRD, and XPS, for the purpose of elucidating the surface corrosion. Then, the reaction mechanism of nZVAl for HBCD degradation was proposed based on the above tests. 2. Materials and methods 2.1. Chemicals Hexabromocyclododecane (HBCD, CAS:3194-55-6) was purchased from Aladdin Reagent Co. Ltd., and trans, trans,cis-cyclododecatriene (t,t,c-CDT) was purchased from TCI (Shanghai) Development Co., Ltd. Commercially available nanoscale zerovalent aluminum (nZVAl, ~50 nm) was purchased from Beijing Dk Nano technology Co., Ltd., without any pretreatment or surface cleaning before used. Methanol, ethanol, and hexane of HPLC grade were all purchased from Merck KGaA, Darmstadt, Germany. NaBr, H2SO4, HCl, NaOH of analytical reagent grade were all purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Water used in all experiments was double distilled. 2.2. Experimental procedures All the experiments were conducted in 100 mL sealed serum bottles to prevent external gases from entering system. A rotary shaker at 250 rpm is needed to keep the nZVAl particles suspending evenly at room temperature (25 ± 1  C). To exhaust the oxygen before the reaction, nitrogen bubbling for 15 min at a rate of 600 mL min1 was needed for all the solutions. The stock solution of HBCD (1.0 g L1) was prepared by dissolving HBCD with ethanol. In a typical reaction system, 2.0 mg L1 HBCD was reduced by 4.0 g L1 nZVAl in ethanol/water (v/v, 50/50) solution without pH adjustment (pH ¼ 5.87) at 25  C. Ethanol was added to increase the solubility (Tso and Shih, 2014; Li et al., 2017a, 2017b), as the concentration of HBCD in saturated aqueous solution was too low to be detected by the instrument. pH was adjusted by 10% H2SO4 and 10% NaOH when it was needed before purging with nitrogen. Sample aliquots were withdrawn at regular time (0, 0.5, 1, 2, 4, 6, 8, 10 and 12 h) intervals, filtered through a 0.22 mm Nylon66 filter. The filtrate was then used for the detection of HBCD or products. For the determination of HBCD, 0.1 mL filtrate was diluted with 1.9 mL ethanol before it was detected by LC-MS. For the analysis of the reductive products, the filtrate was exacted with hexane, and the resulting extracts were then detected by GC-MS. One aliquot of the filtrate was blown to dryness and an appropriate amount of distilled water was added to dissolve the Br for determination. 2.3. Analytical methods HBCD was quantified on a liquid chromatography-mass spectrometry (LC-MS) system (LC, Thermo Scientific U-3000; MS, AB SCIEX Q-TRAP-4500) equipped with an Eclipse Plus C18 RRHD column (1.8 mm particle size, 50  2.1 mm i.d., Agilent). The mobile phase consisted of water (solution A, 10%) and methanol (solution B, 90%), the injection volume was set at 5 mL and the flow rate at 0.2 mL min1. Mass spectrometry analysis was operated in the negative ESI mode with the selection of [M  H]- ion as parent ion. Multiple reaction monitoring (MRM) was implemented. The transitions of m/z 640.6 / 79.0 and m/z 640.6 / 81.0 were selected for

Y. Jiang et al. / Chemosphere 244 (2020) 125536

the qualitative and quantitative determination of HBCD. Sample analysis of reductive products and CDT was carried out using a gas chromatographic-mass spectrometry (GC-MS) system (Agilent HP6890Ne5975 B) equipped with an HP-5MS capillary column (30 m  0.25 mm, 0.25 mm film thickness). The GC oven temperature was maintained at 40  C for 3 min, then programmed from 40  C to 260  C at 20  C min1 (held for 5 min), and to a final temperature of 290  C at 10  C min1 (held for 5 min). Helium was used as the carrier gas and the flow rate was set at 1.2 mL min1. The temperature of the injector was set to 250  C. MS analysis was performed in electron impact ionization mode, both ion source temperature and interface temperature were held at 250  C. The chromatograms were monitored in scan mode for the determination of all degradation products. External standards were used for the quantification of CDT, and the mass used in the MS (54.0, 67.0, 79.0, 93.0, 105.0, 119.1, 133.0, 147.0, 161.8) was confirmed according to the mass spectrum of standards. Beyond that, Br was determined by ion chromatography (ICS3000, DIONEX, USA). The pH of the reaction solution was determined by a pH meter (INESA, China) before purging with nitrogen. 2.4. Characterization of nZVAl particles before and after reaction Scanning electron microscopy (SEM, Hitachi S-4800, Japan) equipped with an energy dispersive X-ray spectroscopy (EDS, QUANTAX 400, Bruker, Germany) was used to observe the nZVAl particles and determine the composition. The morphology and internal structure of the powders were investigated using a transmission electron microscope (TEM, FEI Tecnai G2 TF20ST) and highresolution transmission electron microscopy (HRTEM, JEM2010FEL). X-ray diffraction (XRD) was carried out using an X-ray diffractometer (PW-1830, Philips, Netherlands) with a copper target tube radiation (Cu Ka) to analyze the crystalline structure of nZVAl. To obtain the surface composition of nZVAl, X-ray photoelectron spectroscopy (XPS) analysis was performed on the RBD upgraded PHIe5000C ESCA system (PerkinElmer) with normal Al Ka radiation (hn ¼ 1486.6 eV). 3. Results and discussion 3.1. Degradation and degradation pathway of HBCD by nZVAl 3.1.1. Degradation of HBCD by nZVAl The degradation efficiency of HBCD by nZVAl is shown in Fig. 1.

Fig. 1. Efficiency of HBCD degradation by nZVAl. ([HBCD]0 ¼ 2 mg L1, [nZVAl]0 ¼ 4 g L1, 25  C, pH ¼ 5.87).

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HBCD was completely removed within 8 h reaction when 2.0 mg L1 HBCD reacted with 4.0 g L1 nZVAl at 25  C. During the reaction, a 2 h delay was found (Fig. 1a) at the starting of the reaction which was identified as induction period (Bokare and Choi, 2009). The existence of the induction period was due to the passive oxide film on the surface of nZVAl. The film should be corroded first so that the metallic aluminum inside is exposed to release electrons, or else the reductive capacity of nZVAl will be limited (Zhang et al., 2018). And the time is needed for the corrosion of the oxide film, because the time is needed to break AleOeAl bonds in the oxide film and replaced by AleOH bonds (Reaction (1)) (Deng et al., 2007, 2010), for that which led to the existence of the induction period. Al2O3 þ H2O / 2AlOOH

(1)

3.1.2. Debromination and products of HBCD The yield of Br was determined to be about 90% (Fig. 1), and the debromination efficiency increased as the initial HBCD concentration decreased (Fig. S1). Nearly 100% debromination could be achieved when the initial HBCD concentration was set at 0.2 mg L1, as the less competition of individual HBCD for the released electrons and reactive sites on the surface of nZVAl (Li et al., 2017a, 2017b). The high debromination efficiency indicated that using nZVAl was one of the high efficiency methods for HBCD removal by comparing it with other typical HBCD removing methods, which have been summarized in Table 1. From the liberation of Br (Fig. 2) we can see that the summation of the bromine in remained HBCD and in the solution cannot achieve 100%, which suggested that there were less brominated compounds formed during the debromination of HBCD. In order to identify the debrominated compounds, GC-MS was used to analyze the samples over different time periods during the 12 h reaction according to the analytical method mentioned in section 2.3. As shown in Fig. S2, two compounds were confirmed. The peak at 9.903 min was identified to be CDT, the complete debromination product of HBCD, according to the retention time and mass spectra of CDT standards. As we can see from Table 1, the CDT was not easy to be formed in many removing systems. However, in this study, the yield of CDT was about 67% after 12 h reaction (Fig. 1) without any other energy addition. The peak at 13.553 min (Fig. S2) was identified to be dibromocyclododecadiene (DBCD) by comparing the mass spectra with those reported in the literature (Tso and Shih, 2014; Li et al., 2016). Some scholars reported the finding of tetrabromocyclododecene (TBCD) during the process of reductive debromination, but no definitive evidence was found in this study. This finding is in accordance with the results of HBCD degradation using zero-valent iron nanoparticle aggregates (Tso and Shih, 2014). For further confirmation, LC-MS was used to determine the area change of TBCD (480.8 / 79.0, 81.0) or DBCD (321.0 / 79.0, 81.0) (See Text S1 in Supplementary information for more details). The area of the peak representing DBCD increased and no peak representing TBCD was found during 12 h reaction (Fig. S3). Unfortunately, these compounds could not be quantified because of the lack of authentic standards. 3.1.3. Degradation pathway of HBCD by nZVAl Based on the analysis of HBCD degradation products, a tentative pathway for reductive degradation of HBCD by nZVAl is proposed (Fig. 3). HBCD is found to be degraded to dibromocyclododecadiene (DBCD) and then CDT through reductive debromination by nZVAl. And such a pathway is believed to be also operative in the reductive degradation of HBCD by zero-valent iron nanoparticle aggregates

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Table 1 Summary of typical HBCD removing system. Type

System

e

Condition [HBCD]0

Time

Media

Results f

HBCD

Reference g

Br

h

CDT

Physical

Cu and Fe based MOF

10 mg L1

5h

50% methanol/water

80%

NT

NT

Microbial degradation

Bacillus cereus Sewerage sludge Sphingobium chinhatense IP26

0.5 mM 500 mg L1 1.56 mM

4d 12 d 144 h

100% 92.4% 20% ~ 78%

NT 86% NG

NT ND NG

Soil under various conditions

60 mg kg1

Mineral salt medium Medium Bacteria culture in tris buffer Soil

Li et al. (2017a, 2017b) Shah et al. (2018) Peng et al. (2018) Heeb et al. (2017)

29% ~ 60%

NT

NT

Le et al. (2017)

15 W UV-lamp; lmax: 220 e260 nm simulated sunlight 500  C mixed with aluminum

7.79 mM

21 e40 d 40 min 40% acetonitrile/water

100%

NG

NG

Yu et al. (2015)

80 min Aqueous solutions 1.0 mM 200e300 mg 30 h Pure nitrogen

54.8% 100%

ND 58% ~ 65%

NT NG

200  C; with Fe3O4 addition nZVI: 5 g L1 S-nZVI: 0.5 g L1

779.2 nM 20 mg L1 20 mg L1

FeS: 5.5 mg L1 Polysulfide and bisulfide nickel salen; glassy carbon cathode FeeQuartz (10:1): 5.5 g Sodium persulfate and NaOH 25 W LED lapm; oven (120  C) nZVAl

2 mg L1 24 h 100e500 mM 25 h 2e20 mM NG

50% ethanol/water 80% methanol/water DMF containing TMABF4

0.5 g 0.5 g i 0.1 phr 0.2 e20 mg L1

Soil Solid Polystyrene 50% ethanol/water

Photolysis degradation Thermal degradation a

Reductive

b

Electro-

c

Mechano-

d a

Hybrid Reductive

1h Gaseous 30 min 50% methanol/water 12 h 50% ethanol/water

2h 2h 20 h 8h

Zhou et al. (2014) Barontini et al. (2003) 92.85% ~ 100% 89.15% NG Wu et al. (2019) 100% NT NG Tso and Shih (2014) 100% NG ND Li et al. (2017a, 2017b) 90% 50% NG Li et al. (2016) 80% NG ND Lo et al. (2012) 100% NT 35% ~ 88% Wagoner et al. (2014) 100% 96.7% NG Zhang et al. (2014) 95% 100% NT Yan et al. (2017) 94% NT NT Arita et al. (2017) 100% 56.7% ~ 104.7% 67% This study

ND: not detected; NT: not tested; NG: not given. a Reductive degradation. b Electrochemical degradation. c Mechanochemical degradation. d Hybrid system. e The unit of percentage is v/v. f The percentage of HBCD removed. g The yield of Br. h The yield of cyclododecatriene (CDT). i Per hundred resin.

nZVAl is shown in Fig. 4a and the reaction rate increased accordingly as the dosage of nZVAl increased from 0.4 g L1 to 8.0 g L1. This was because more reactive sites were provided and plenty of electrons were released with the dosage increased (Ren et al., 2018). Hydrolysis experiments of HBCD were carried out under different pH conditions (pH ¼ 5.87 without adjustment, pH ¼ 9.00, pH ¼ 10.00) without adding nZVAl. Similar to that reported in the literature (Zhang et al., 2019), no significant hydrolysis of HBCD was observed (Fig. 4a). This means that although the pH value of the final reaction solution varied from 8.37 to 9.48 (Fig. S4a) as the dosage of nZVAl increased, hydrolysis does not account for the degradation of HBCD.

Fig. 2. The liberation of Br on the HBCD degradation by nZVAl. ([HBCD]0 ¼ 2 mg L1, [nZVAl]0 ¼ 4 g L1, 25  C, pH ¼ 5.87).

(Tso and Shih, 2014). 3.2. Degradation under different conditions 3.2.1. Effect of dosages of nZVAl The degradation performance of HBCD by different dosages of

3.2.2. Effect of dissolved oxygen The presence of dissolved oxygen (DO) can compete to get electrons and leads to the formation of oxide film (Yang et al., 2017). The effect of DO was carried out by comparing the result of degradation with or without purging with nitrogen at the beginning of the reaction. As shown in Fig. S5, the existence of oxygen has a little influence on the performance of HBCD degradation. Although the induction period was prolonged and the reaction rate slowed down, there was still about 84% HBCD degraded during the 12 h reaction. The results can be explained by the following reasons. On the one hand, the oxide film reformed when the metallic aluminum inside contact with DO (Ren et al., 2018), leading to the time extension of the induction period and degradation; on the other hand, although the releasing of electrons was influenced by the reformation of the film on the surface, the amount of electrons

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Fig. 3. The proposed degradation pathway of HBCD by nZVAl.

Fig. 4. Effect on HBCD degradation: (a) dosages of nZVAl; (b) temperature; (c) inorganic ions; (4) initial pH. (When the effect of a given factor was examined, the other factors were kept constant: [HBCD]0 ¼ 2 mg L1, [nZVAl]0 ¼ 4 g L1, 25  C, pH ¼ 5.87).

was still large enough for HBCD degradation.

3.2.3. Effect of temperature Temperature has a significant influence on the reaction involved in nZVAl (Gai et al., 2012). As the results described in Fig. 4b, the time for complete HBCD degradation was shortened sharply with the temperature elevating from 18  C to 45  C. What’s more, HBCD can be 100% degraded in only 40 min with elevating the reaction temperature to 45  C. This suggested that the higher reaction temperature can strongly promote HBCD degradation by nZVAl.

3.2.4. Effect of Cl and SO24 The effect of chloride ions (Cl) or sulfate ion (SO2 4 ), which are common in the water system, on the degradation of HBCD has been investigated, respectively. Both the effect of different kinds of inorganic ions and the effect of ionic concentration are shown in Fig. 4c. The existence of these two ions, used as sodium salt form, slowed down the degradation of HBCD by nZVAl to some extent.  The effect of SO2 4 was stronger than Cl when the same concentration was used in the system, which can be due to their different properties. It was reported that, in contrast to Cl, SO2 4 was doubly charged, hence had a stronger adsorptive affinity for aluminum surface (Boukerche et al., 2014). Because of the pitting corrosion of

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Cl on nZVAl (McCafferty, 2003), the induction period was shortened to 1 h with the addition of Cl. It has been considered that Cl may destroy Al-(hydr)oxide layer to a certain extent, and thus nZVAl would be corroded more effectively (McCafferty, 2003). Beyond that, the ionic concentration, over the range of 0.01e0.10 mol L1, had some influence on the HBCD degradation (Fig. 4c). For the same inorganic ion, as the ionic concentration elevated, the rate of reaction slowed down and the efficiency of the degradation decreased. This might be attributed to the competition between the ions and HBCD for the available reaction sites of the nZVAl and the result was in coordination with previous research (Li et al., 2017a, 2017b). 3.2.5. Effect of initial pH Solution pH is one of the most important characteristics that influence the rate of degradation since the change of pH affects nZVAl corrosion greatly. It has been reported that the corrosion of oxide film occurred rapidly under pH < 4.00 or pH > 8.60 following Reactions (2) and (3), respectively (Deltombe and Pourbaix, 1958; Dai et al., 2011). What’s more, the corrosion of passive oxide film is slower under acidic conditions than under alkaline conditions (Boukerche et al., 2014). And if the film of oxide is eliminated, the attack is immediate, both in acid and in alkaline solutions (Deltombe and Pourbaix, 1958). Al2O3 þ 6Hþ / 2Al3þ þ 3H2O

(2)

Al2O3 þ H2O þ 2OH / 2Al(OH)-4

(3)

The degradation results at different pH are presented in Fig. 4d. Compared with pH ¼ 5.87 (normal condition, pH without adjustment), the rate of degradation was a little faster in alkaline solutions (pH ¼ 11.00, 9.00). What’s more, HBCD was slightly degraded during the first 2 h indicating that the passive oxide film was corroded a little faster under pH ¼ 9.00 and pH ¼ 11.00 (Yang et al., 2017). Although the induction period prolonged to 6 h under pH ¼ 4.00, after that, the degradation of HBCD proceeded quickly as well. The reason for the different induction period under different pH may be due to the change of pH during the reaction (Fig. S4b). When the initial pH ¼ 11.00, 9.00, 5.87, the pH all changed to pH value 9.00 nearby (pH ¼ 9.27, 8.91, and 8.76, respectively) after 2 h. Then the pH was maintained at this level, which was in favor of the corrosion of film (Deltombe and Pourbaix, 1958). A similar change rule was also found under initial pH ¼ 4.00, but it seemed that much more time (6 h) was needed to achieve pH value 8.78. Because the Al-(hydr)oxide produced by the corrosion has poor solubility at pH ¼ 5.1 nearby (Deltombe and Pourbaix, 1958) and covers the surface of the nZVAl. The progress of the reaction slows down and thereby leading to the prolongation of induction period. 3.3. Surface corrosion of nZVAl during HBCD degradation 3.3.1. Morphology and elemental analysis Morphology and elemental analysis of nZVAl were carried out through SEM-EDS, TEM and HRTEM analysis. Both the SEM image (Fig. 5a1) and TEM image (Fig. 5a2) displayed that the fresh nZVAl

Fig. 5. 1SEM, 2TEM and 3HRTEM of nZVAl reacted with HBCD at different time: (a) before reaction; (b) 4 h; (c) 8 h; (d) 10 h ([HBCD]0 ¼ 2 mg L1, [nZVAl]0 ¼ 4 g L1, 25  C, pH ¼ 5.87).

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before reaction was a spherical morphology with good dispersibility, and the surface of nZVAl particles was smooth and uniform. Furthermore, as the HRTEM image (Fig. 5a3) showed obviously, nZVAl powers had a core/shell structure and the surface was coated with a layer of 2e3 nm film, which is the passive oxide film (Yang et al., 2017). After a 4 h reaction, the shape of nZVAl did not change a lot, but the particles became rough and irregular floccules formed covering on the surface (Fig. 5b1 and Fig. 5b2). As the reaction continued, the oxide film became loosen. Furthermore, the boundary between the shell and the core of ZVAl became irregular (Fig. 5b3). With the reaction proceeding, the rough surface was obviously observed (Fig. 5c1 and Fig. 5c2), a lot of floccules appeared and entangled with each other (Fig. 5d1 and Fig. 5d2). The boundary of core-shell still existed (Fig. 5c3 and Fig. 5d3), and this phenomenon is consistent with the statement of our earlier research (Yang et al., 2017). These phenomena indicated that the corrosion was occurred on the oxide film, making the surface of nZVAl rough and loose. Metallic aluminum was exposed and the reactive sites were provided for HBCD degradation. The main elemental composition was analyzed by EDS (Table S1). Al and O were the main elements existing on the surface of nZVAl. After the reaction, the content of Al decreased from 88.27% (original nZVAl) to 42.90% (10 h reaction) while the content of O increased from 11.73% to 51.70%. It illustrated clearly that the oxidation of nZVAl occurred and electrons were yielded after the surface film was corroded.

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As we all know, the dense oxide film on the surface of nZVAl protects the metallic aluminum inside from corrosion, but it also hinders the release of electrons, which in turn affects its application to the degradation of pollutants. During this reaction, the oxide film was corroded firstly, and Al2O3 was converted into Al-(hydr)oxide at the first 2 h. The smooth and uniform film was changed to be rougher one with floccules, and that was believed to be favorable for the removal of the contaminants (Yang et al., 2017). Then, metallic aluminum was exposed and electrons were released, in the meantime, the reactive sites were provided for electrons tunneling. Due to the high bromine content in HBCD, reductive debromination played the main role in the degradation reaction. The reactive sites were occupied by HBCD, and then, the electrons released were transferred from metallic aluminum to HBCD inducing the reduction reaction, thus HBCD was degraded to DBCD via debromination, and then CDT. 4. Conclusion In this study, we have explored the performance and mechanism of HBCD reductive degradation using nZVAl in detail. The

3.3.2. XRD analysis XRD analysis shows the composition evolution and crystalline change of nZVAl powers before and after reaction. The XRD analysis is shown in Fig. S6, the peaks at 38.5 , 44.7, 65.1, 78.2 , 82.5 , corresponding to (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes of face centered-cubic, was sharp and intense (Guo et al., 2007). No significant change of these peaks and no other peaks can be observed after 10 h reaction, which suggested that the crystal phase of nZVAl was stable and the oxidation products phases may exist in the amorphous phase during the reaction (Yang et al., 2017). 3.3.3. XPS analysis XPS analysis was carried out to explore the surface corrosion deeper, and the XPS patterns of Al 2p and O 1s are displayed in Fig. 6. There were three peaks before the reaction belonging to Al (Fig. 6a). After 10 h reaction, the binding energy at 71.4 ± 0.2 eV, assigned to the metallic contribution of aluminum (Alexander et al., 2010), nearly disappeared according to the detailed information displayed in Table S2. While the area ratio of the other two peaks at 73.68 ± 0.2 and 74.36 ± 0.2 eV (Alexander et al., 2000) were increased, which were both assigned to Al-(hydr)oxide structure. These data suggested direct evidence that, after the corrosion occurred on the surface of nZVAl, the metallic aluminum was oxidized and electrons were released that could degrade HBCD. For the XPS spectra of O 1s (Fig. 6b), the peaks at 530.65 ± 0.2 eV, 531.90 ± 0.2 eV assigned to O2 and OH, respectively (Alexander et al., 2000, 2010). By contrasting the data before reaction and after 10 h reaction, the area ratio of O2 decreased from 52.23% to 34.08% while OH increased from 47.77% to 65.92%, which indicates that the content of Al oxide and hydroxide had some changes during the reaction. And it also supplied further evidence for the formation of Al-(hydr)oxide (Al2O3, AlOOH or Al(OH)3) on the surface during the corrosion of nZVAl. 3.4. The proposed reaction mechanism According to the degradation of HBCD and the surface corrosion of nZVAl, the reaction mechanism is proposed as follows.

Fig. 6. (a) XPS of nZVAl before reaction and after 10 h reaction. ([HBCD]0 ¼ 2 mg L1, [nZVAl]0 ¼ 4 g L1, 25  C, pH ¼ 5.87).

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results suggested that: 1) HBCD could be highly reduced. Nearly 100% debromination can be obtained under optimized conditions and the yield of CDT was one of the highest compared to the published research results now available. The performance was significantly enhanced with the nZVAl dosages increasing, initial HBCD decreasing, temperature elevating and oxygen removing. 2) Identification of the degradation products indicated that, HBCD could be reductively debrominated to DBCD via acquiring electrons from nZVAl, and then CDT, the complete debromination product. 3) The core/shell structured nZVAl participated in the reaction. The surface oxide film was corroded at the starting of the reaction and the metallic aluminum inside exposed, and then reactive sites were provided and electrons were released for HBCD degradation. These findings indicated that HBCD can be reductively debrominated in high efficiency using nZVAl in ethanol/water (v/v, 50/ 50) solution. Thus, a way for HBCD degradation was provided, which was proved to be rapid and efficient. This study provides a new idea for the better treatment of HBCD. Author statement Yuting Jiang: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Resources, Data Curation, Writing Original Draft, Writing - Review & Editing, Visualization; Shiying Yang: Resources, Conceptualization, Writing - Review & Editing, Supervision, Project administration, Funding acquisition; Junqin Liu: Methodology, Validation, Investigation; Tengfei Ren: Methodology, Investigation; Yixuan Zhang: Methodology, Investigation; Xinrong Sun: Investigation. Declaration of competing interest There are no known conflicts of interest. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 21677135). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemosphere.2019.125536. References Alexander, M.R., Thompson, G.E., Zhou, X., Beamson, G., Fairley, N., 2010. Quantification of oxide film thickness at the surface of aluminium using XPS. Surf. Interface Anal. 34 (1), 485e489. https://doi.org/10.1002/sia.1344. Alexander, M.R., Thompson, G.E., Beamson, G., 2000. Characterization of the oxide/ hydroxide surface of aluminium using X-ray photoelectron spectroscopy: a procedure for curve fitting the O1s core level. Surf. Interface Anal. 29, 468e477. https://doi.org/10.1002/1096-9918(200007)29:7<468::aid-sia890>3.0.co;2-v. Arita, S., Yamaguchi, K., Motokucho, S., Nakatani, H., 2017. Selective decomposition of hexabromocyclododecane in polystyrene with a photo and thermal hybrid treatment system. Polym. Degrad. Stab. 143, 130e135. https://doi.org/10.1016/ j.polymdegradstab.2017.07.003. Arslan-Alaton, I., Olmez-Hanci, T., Ozturk, T., 2018. Effect of inorganic and organic solutes on zero-valent aluminum-activated hydrogen peroxide and persulfate oxidation of bisphenol A. Environ. Sci. Pollut. Res. 25 (35), 34938e34949. https://doi.org/10.1007/s11356-017-1182-9. Barontini, F., Cozzani, V., Petarca, L., 2003. The influence of aluminum on the thermal decomposition of hexabromocyclododecane. J. Anal. Appl. Pyrolysis 70 (2), 353e368. https://doi.org/10.1016/S0165-2370(02)00182-1.

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